Illumination system for displaying images without color break

ABSTRACT

The invention concerns an image display system comprising: a light source emitting an illuminating laser beam, a spatial light modulator controlled by control video signals corresponding to a succession of image frames; a matrix filter consisting of an array of different elementary color filters, illuminated by said illuminating beam and transmitting a spatially color-filtered beam to the spatial light modulator, an image of said filter being produced on an input surface of the spatial light modulator; means for displacing the filter image on the input of the spatial light modulator; and a control device for controlling at least one sequence of displacements of the filter image upon each image frame display.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/EP03/50708, filed Oct. 13, 2003, which waspublished in accordance with PCT Article 21(2) on May 6, 2004 in Frenchand which claims the benefit of French patent application No. 0213981,filed Oct. 28, 2002.

BACKGROUND OF THE INVENTION

The invention relates to a system for displaying images with the aid ofa spatial light modulator.

SUMMARY OF THE INVENTION

It applies more particularly to mono-display video back-projection orprojection systems, with matrix displays and more particularly tosystems employing a relay image upstream of the projection screen. Itaims to improve the image quality for color sequential systems.Additionally, it makes it possible to degrade a video obtained with acamcorder that recorded the image projected on the projection screen.

Two types of mono-imager architectures are known:

-   -   a—that where all the pixels (image elements) of the imager        always see the same color: “all red”, “all green”, or “all        blue”; this is what is obtained by using a color wheel which        rotates in front of the imager. This mode is dubbed “color        sequential”;    -   b—that where the scanning of the imager takes place “line by        line” (case of “color scrolling”). All the pixels of a group of        lines of the imager see the same color, so that, for each line,        there is in succession an “all red” line, an “all green” line        and an “all blue” line. This effect can be obtained by using a        rotating filter having helical striped color bands.

Projection or back-projection systems can therefore arise in variousconfigurations. The invention relates to mono-display configurations,operating in color sequential mode and able to employ a plane accessibleupstream of the main imager and optically conjugate with the latter.

It is known, for example, from the document US2002/0024618, that suchsystems operating in color sequential mode give rise to risks of colorbreak-up during projection, which cause the appearance of multiple andcolored contours on the images displayed, when these images are movingor when the observer shifts his gaze over the projected image. Thesecolor break-ups stem from the fact that the three primary imagesub-frames, red, green and blue, which, after integration by the eye,form one and the same overall polychrome image, are displayed one afterthe other and hence at different instants.

Document US2002/0024618 proposes a solution to this problem, in the casewhere each polychrome image to be displayed is distributed, not into asequence of three monochrome sub-frames, red, green and blue, but into asequence of four sub-frames, red, green, blue and white. This documentproposes that the pixels or cells of the matrix display be grouped intogroups of four adjacent pixels, and that the display be illuminated insuch a way that the four pixels of each of the groups are illuminated bydifferent colors, one in red, a second in green, a third in blue, andthe fourth remaining in white. At each image sub-frame, polychromeillumination is therefore carried out which forms on the entrance faceof the matrix display a mosaic of color “patches”, each patch ofhomogeneous color corresponding to a pixel. The polychromatic image tobe displayed is then composed sequentially by alternating the color ofillumination of each of the pixels within each group. In this way, asindicated in paragraph 19 of this document, the colors illuminating theadjacent pixels of one and the same sub-frame are mixed by juxtaposedadditive synthesis; as each sub-frame is no longer, as before,monochrome, the observer no longer perceives color break-ups when hisgaze shifts over the image or when the image is moving.

The invention proposes an enhancement to the general solution taught bythe document US2002/0024618 which makes it possible to obtain, in a muchsimpler manner than in the embodiments described in this document,polychrome illumination of the matrix display during each sub-frame:specifically, to obtain this polychrome illumination, it is proposed

-   -   to use a matrix filter formed of a mosaic of elementary        monochrome filters, to illuminate this filter by a generally        white polychrome source, and to make the image of this filter        thus illuminated on the entrance face of the spatial modulator,    -   to use means for displacing the image of this filter from one        sub-frame to the next, in such a way as to alternate the color        of illumination of each of the pixels or set of pixels of this        modulator.

The invention thus makes it possible to solve the problem of colorbreak-up in a much simpler manner than in the prior art.

The invention therefore relates to a system for displaying images withthe aid of a spatial light modulator comprising:

-   -   a light source emitting an illumination beam;    -   a spatial light modulator comprising a matrix of pixels        controlled by video control signals corresponding to a        succession of image frames to be displayed;    -   a matrix filter formed of a mosaic of elementary filters of        various colors, illuminated by said illumination beam and        transmitting a spatially filtered color beam to the spatial        light modulator,    -   means for producing an image of said filter on an entrance face        of the spatial light modulator;    -   means of displacement for displacing the image of the filter on        the entrance face of the spatial light modulator and    -   a device for controlling these means of displacement, making it        possible to control at least one sequence of displacements of        the image of the filter during each image frame.

The control device is adapted to control the displacements of the imageof the filter in synchronism with the video control signals of thespatial light modulator.

Preferably, each displacement of a sequence corresponds to a multiple ofthe dimension of the image of an elementary filter on the entrance faceof the spatial modulator.

Preferably, the dimensions and the position of the elementary filtersare adapted so that the image of each of them on the entrance face ofthe spatial modulator covers a plurality of pixels. The dimensions ofeach elementary filter are then such that they allow the illumination ofa number of pixels of the spatial light modulator which is an integergreater than one. That is to say each elementary filter is adapted so asto simultaneously illuminate several adjacent pixels during each imagesub-frame.

One of the drawbacks of the various spatial modulator illuminationdevices described in document US2002/0024618 is that they must beadapted to obtain, during each image sub-frame, illumination of variouscolors over adjacent pixels, which in practice turns out to be extremelydifficult to obtain and to maintain on account of the small size of thepixels of the display. The invention makes it possible to avoid thisdrawback.

In practice, matters are preferably contrived so that the limits of theimage of each elementary filter correspond to inter-pixel spaces on theentrance face of the modulator; color mixtures within one and the samepixel are thus avoided. Each elementary filter then makes it possible toilluminate the totality of several pixels.

The mosaic may be monodimensional, in the sense that it includes forexample only one column of elementary filters of various colors; eachelementary filter then forms a monochrome colored band extending overthe entire width of the filter. During an image sub-frame, all thepixels of a group of rows of the spatial modulator then simultaneouslysee the same color. During a succession of sub-frames, each row ofpixels is successively illuminated in red, in green and in blue. Theinvention then makes it possible to obtain in a very simple manner ascrolling of color bands over the spatial modulator.

Preferably, in order to better solve the abovementioned problem of colorbreak-up, the mosaic is bidimensional and the monochrome elementaryfilters are arranged in several rows and several columns; if the spatiallight modulator comprises a bidimensional matrix of pixels each formedby an optical valve and arranged in rows and columns, the direction ofthe image of the rows of elementary filters on the entrance face of themodulator corresponds to that of the rows of pixels, and the directionof the image of the columns of elementary filters on the entrance faceof the modulator corresponds to that of the columns of pixels; theoptical valves may be liquid crystal cells or micromirror elements.

According to a preferred embodiment of the invention, said mosaic isformed by the repetition of blocks of elementary filters, these blocksexhibiting identical contours and each being composed of at least twoelementary filters of different colors; since all the blocks have thesame contours, that is to say the same geometry, each block thereforecomprises the same number of elementary filters; in the filter, thedistributions of the elementary filters of different colors in theblocks may be different from one block to another. Preferably, eachblock comprises three elementary filters: one red, one green and oneblue.

According to another variant embodiment of the invention, there isprovision for a block to comprise more than two filters, which areadjacent but not aligned.

According to another variant embodiment of the invention, there isprovision for a block to comprise more than two filters which areadjacent and aligned. Preferably, these blocks are then arranged in sucha way that the elementary filters of like color are aligned along adirection tilted with respect to that of the rows and that of thecolumns of elementary filters. During the design of a filter, suchblocks will therefore be offset with respect to one another so as toobtain patterns in which the elementary filters of like color arealigned along tilted directions; preferably, two rows will then bemutually interchanged and/or two columns will then be mutuallyinterchanged. Such a filter will be easy to design and to use whilescrambling the pattern formed by the groups of blocks.

Preferably, the filter comprises the same number of elementary filtersof each color in the various rows and in the various columns of thefilter.

Provision may also be made for the mosaic to be an assemblage ofidentical patterns each comprising the same number of blocks and thesame number of elementary filters of each color in each of the rows andin each of the columns of elementary filters of this pattern. This willmake it possible in a more dependable manner to obtain a white image foreach pixel of the spatial light modulator which is in the on state.Preferably, the means of displacement are adapted for displacing theimage of the mosaic-like filter transversely to the direction of theillumination beam. According to one embodiment, the means ofdisplacement comprise a light deflection device, located between thematrix filter and the spatial light modulator; this device is adaptedfor displacing the image of the filter over the entrance face of themodulator; the control device thus the deflection, by the deflectiondevice, of the spatially filtered illumination beam, thus yieldingdisplacements of the image of the filter over the entrance face of thespatial light modulator.

Advantageously, the deflection device comprises an orientable mirror;the matrix filter and the spatial light modulator are then arrangedsymmetrically with respect to a beam splitting surface; the system thencomprises an imaging optic receiving the light emitted by the matrixfilter, retransmitting it to the mirror which reflects it toward thesplitting surface via the imaging optic, which splitting surfacereflects the light toward an entrance face of the spatial lightmodulator, an image of the matrix filter thus being formed on theentrance face of the spatial light modulator, this image beingdisplaceable over this entrance face by rotation of the orientablemirror.

The displacements indicated above allow displacement of the image of thefilter on the spatial light modulator in such a way that each sequenceof displacements of the image of the filter on the entrance face of thespatial light modulator allows the successive illumination of each pixelof the spatial light modulator by all the elementary filters of one andthe same block. Hence, this makes it possible to color an image of thespatial light modulator.

Moreover, provision may be made, during each image frame, for each pixelof the spatial light modulator to be illuminated successively by all theelementary filters of a block under the effect of a first sequence ofdisplacements, then by all the elementary filters of another block underthe effect of a second sequence of displacements.

According to this variant, piracy with the aid of camcorders may berendered even more difficult through the invention. Specifically, theinvention will make it possible to display, with a random sequence,images exhibiting colored structures when they are displayed by acamcorder. These colored structures are not visible to the eye on theimage projected from the spatial modulator, because the eye produces ananalog sliding average of the various sub-frames. These coloredstructures will on the other hand be visible on the video tape of thecamcorder that recorded the projected image, owing to the fact that thetemporal sampling carried out by the camcorder may no longer correspondto the temporal sampling of the image sub-frames displayed by thespatial modulator of the system according to the invention. Thescrambling of the video image emanating from the camcorder may deter thecommercialization of such a pirated video.

Preferably, all the sequences of displacements controlled by the controldevice are adapted so that the integration of the images of the filterthat are obtained over the set of displacements of the sequence orsequences of each frame imparts a white colorimetry to the entrance faceof the spatial light modulator. If each frame comprises just a singlesequence, each sequence imparts on its own a white colorimetry. If eachframe comprises a combination of sequence, each combination of sequencesimparts a white colorimetry without, however, each sequence impartingonly a white colorimetry.

In the case where each frame comprises a first sequence and at least onesecond sequence, these sequences are then preferably adapted so that theintegration of the images of the filter that are obtained over the setof displacements of any one of these sequences imparts a nonwhitecolorimetry to the entrance face of the spatial light modulator; as onlythe succession of several sequences imparts a white colorimetry, such anarrangement will lead to the impairment of the images of the spatiallight modulator that are filmed by a camcorder.

To more effectively prevent piracy by a camcorder, preferably, thecontrol device possesses the characteristics of various combinations ofat least two sequences of displacements, chosen from among a plurality,each combination making it possible to impart a white colorimetry of theentrance face of the spatial light modulator. The control device thenselects, from among these combinations, different combinations forsuccessive frames. It is not vital to change combination between eachframe, but only between certain frames, which may be chosen at random.Preferably, the selection of combination from among the plurality isalso random.

Moreover, provision may be made for said control device to possess thecharacteristics of a plurality of different sequences of displacementsmaking it possible to impart a white colorimetry to the entrance face ofthe spatial light modulator and for this device to select, from amongthis plurality, different sequences for successive frames. If theintegration time of an image recorded by a camcorder overlaps two framesof different sequences, this will advantageously culminate in animpairment of the images of the spatial light modulator that are filmedby this camcorder. It is not vital to change sequence between eachframe, but only between certain frames, which may be chosen at random.Preferably, the selection of sequences from among the plurality is alsorandom.

BRIEF DESCRIPTION OF THE DRAWINGS

The various subject matter and characteristics of the invention willbecome more clearly apparent in the description which follows given byway of nonlimiting example and in the figures which represent:

FIG. 1, a general exemplary embodiment of the system of the invention;

FIG. 2, an exemplary matrix filter applied in the system of FIG. 1;

FIG. 3, an exemplary embodiment of the system of the invention;

FIG. 4, a variant embodiment of the system of the invention;

FIGS. 5 a to 5 f, exemplary embodiments of a filter according to theinvention;

FIGS. 6 a to 6 l, figures making it possible to explain the operation ofthe system of the invention;

FIGS. 7 a to 7 c, of the illustration of camcorder anti-piracyoperation;

FIGS. 8 a to 8 c, a variant embodiment of the filter according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, a general exemplary embodiment of the systemof the invention will therefore be described.

This system comprises a light source preferably emitting a beam of whitelight making it possible to illuminate a spatial light modulator 2. Thisspatial light modulator comprises a set of pixels (image elements)arranged in matrix form and is for example a liquid crystal valve. Afilter 3 makes it possible to spatially filter the various wavelengthscorresponding to the red, green and blue colors so as to illuminate thespatial light modulator 2 with beams of various colors.

A transmission optic 4 makes it possible to image each point of thefilter 3 in substantially the plane of the spatial light modulator 2.Moreover, in the case of a projection or back-projection application, anexit optic 6 makes it possible to configure the beam transmitted by thespatial light modulator.

The filter 3 possesses a set of elementary filters of different colors(that is to say of different wavelength filtering characteristics).Preferably, each elementary filter makes it possible to illuminate aninteger number greater than one of pixels of the spatial lightmodulator.

FIG. 2 represents an example of a filter according to the inventionembodied in the form of a bidimensional matrix, that is to say organizedin rows and columns, of red (R), green (G) and blue (B) elementaryfilters. The distribution of the various elementary filters R, G and Bwill be explained later.

A control device 5 makes it possible to displace the spatial filteringof the illumination beam, this amounting to displacing the image of thefilter 3 on the entrance face of the spatial light modulator 2. As isrepresented in FIG. 1, the control device 5 can control thisdisplacement:

-   -   either by displacing the filter 3 perpendicularly to the        direction of the illumination beam as indicated by the arrow D;    -   or by providing a beam deflection or translation device 7        between the filter 3 and the spatial light modulator 2. For        example, in FIG. 1, a deflection of the beam transmitted to the        spatial light modulator is obtained by rotating the device 7 as        indicated by the arrow R.

The control device 5 thus controls the displacement of the image of thefilter on the entrance face of the spatial light modulator 2. Thisdisplacement is made step by step along two orthogonal directions sothat the image of the filter displaces over the entrance face of thespatial light modulator in two orthogonal directions parallel to therows and to the columns. With each displacement, the displacement pitchis equal to a multiple of the distribution pitch of the images of theelementary filters of the filter 3 on the entrance face of the spatiallight modulator.

Let us consider that the point p2 of the spatial light modulator isilluminated by an elementary filter situated at the point p3 of thefilter.

At an instant t0, the elementary filter located at the point p3 is of adetermined color, red for example and the pixel situated at the point p2of the spatial light modulator is illuminated by red light. At aninstant t1 thereafter, under the control of displacement of the filter 3by the device 5, the elementary filter located at the same point p3 isgreen (for example) and the pixel of the point p2 is illuminated bygreen light. At another instant t2, the elementary filter located at p3may thereafter be blue and the pixel situated at p2 is illuminated byblue light.

The distribution of the elementary filters R, G and B of the filter 3 iseffected in such a way that by providing appropriate displacements ofthe filter, a light is obtained which is on average perceived as whitefor all the pixels of the spatial light modulator when these pixels arein the on state for the various positions of displacements, this beingso for an integration time suitable for the eye.

In the case where the control device gives rise to a displacement of theimage of the filter over the entrance face of the spatial lightmodulator by deflection of the beam transmitted by the filter forexample, operation is similar.

A processing of the synchronous signal will supply the imager with videosignals combining the initial images of the three colors according to apattern identical to that of the colored filters. The control device 5will operate in synchronism with the video signals.

Each sub-image will then contain pixels of the three colors, accordingto a random or pseudo-random pattern, and this will no longer placecontours of colors at different instants but will distribute them overtime. This will attenuate the phenomenon of color break-up.

FIG. 3 represents an exemplary embodiment of a projection system usingthe illumination system according to the invention. In this figure areagain found the light source 1, the filter 3, the optic 4, the spatiallight modulator 2, the exit optic 6, the beam deviation or translationdevice 7 and the control device 5 of FIG. 1.

A light integrator device, which may be embodied in the form of anintegrator bar 10, is interposed between the source 1 and the filter 3so as to provide uniform illumination of the surface of the filter 3 andsubsequently of the surface of the spatial light modulator.

Moreover, in the case, for example, of a spatial light modulatoroperating by reflection, provision may be made for a beam splitter 8associated with the entrance face of the spatial light modulator whoseopposite face is reflecting or is furnished with a reflection device 12.The light originating from the filter is transmitted to the spatiallight modulator which modulates it spatially and reflects it toward thesplitter, which then reflects the light toward the exit optic 6. Itshould be noted that the light polarization means necessary for theoperation of the spatial light modulator are well known in the art andare not represented in the figure.

The control device 5 makes it possible to displace the filter 3 alongtwo perpendicular directions DX and DY contained in a plane transverseto the direction of the beam transmitted by the integrator bar 10 so asto displace the image of the filter over the entrance face of thespatial light modulator. According to a variant embodiment, a beamdeflection or translation device 7 controlled by the device 5 makes itpossible to effect this displacement of the image of the filter over theentrance face of the spatial light modulator.

Referring to FIG. 4, a variant embodiment of a projection systemapplying the illumination system of the invention and which has theadvantage of being compact will be described.

The filter 3 and the spatial light modulator 2 are arrangedsymmetrically with respect to a light splitting surface 19. According tothe exemplary embodiment of FIG. 4, this surface 19 is the splittingsurface of a beam splitter cube 18.

The filter 3 is furnished with a reflection device 13 so that the lightthat it receives from a light source and from an integrator device 10through a beam splitter 11 and through the splitting surface 19 isreflected toward an optic 4 and a mirror 17. The light reflected by themirror 17 is returned by the optic 4 and the splitting surface 19 to thespatial light modulator 2.

The light therefore performs a double pass through the optic 4; thelatter is designed as a double Gauss optic so that by reason of thesymmetric positions of the filter 3 and of the spatial light modulator 2with respect to the splitting surface 19, as well as of the double passof the light through the optic 4, the surface of the filter is imaged onthe entrance surface of the spatial light modulator 2 with magnification1 and without distortion.

As may be seen in FIG. 4, the mirror 17 is movable about twoperpendicular axes X1 and X2. Rotation commands R1 and R2 instructed bythe device 5 make it possible to displace the image of the filter 3 overthe entrance face of the spatial light modulator in two perpendiculardirections, horizontally and vertically in particular.

When the mirror is perpendicular to the direction of the beam that itreceives from the filter, it is in a mean position. Operation in whichthe rotations R1 and R2 cause the mirror to oscillate about this meanposition will be favored.

The spatial light modulator 2 is furnished on its face opposite itsentrance face with a reflection device 12. The light originating fromthe filter 3 and illuminating the spatial light modulator is thereforereflected toward the exit objective 6. As before, the means ofpolarization necessary for the operation of the system are entirelyknown in the art and are therefore not represented in the figure.

Referring to FIGS. 5 a to 5 f, the design of a filter 3 according to theinvention will now be described.

As indicated earlier, this filter comprises a matrix of coloredelementary filters, that is to say ones having different opticalwavelength filtering characteristics. The distribution of the elementaryfilters is such that the filter 3 exhibits a repetition of patterns eachconsisting of a determined number of elementary filters. For example,FIGS. 5 b and 5 c represent a pattern of 3×3 elementary filters andFIGS. 5 d and 5 e represent a pattern of 6×6 elementary filters. It isof course obvious that a pattern could comprise more elementary filters.

A method for obtaining these patterns is as follows; in an m×n matrixwhere m×n is a multiple of three, choose a shape of a block of threeelementary filters for example (in the case of three-color operation)such as the shape of FIG. 5 a to produce the patchwork of FIG. 5 b (orthat of FIG. 5 d).

The distribution of the elementary filters R, G and B may be differentin the various blocks of three elementary filters. Thus, block M1 isdifferent from block M2 as regards the distribution of the elementaryfilters R, G and B.

The colors of the elementary filters are laid out randomly over thevarious shapes while nevertheless preferably complying with overallhomogeneity criteria (e.g.: same number of pixels of each color for therows and the columns of the matrix).

The patterns obtained (FIG. 5 c or 5 e) will be replicated bytranslation so as to cover the totality of the filter 3.

It will be noted that in order to take account of the displacement ofthe image of the filter 3 on the surface of the spatial light modulator2 and in order in all cases for this image to cover the spatial lightmodulator, a filter of larger surface area than that of the spatiallight modulator will be provided. If there are provided translations of±1, ±2 or ±3 elementary filters, the dimensions of the filter will beincreased by rows and by columns corresponding to one to three rows andone to three columns of elementary filters of the pattern in eachdirection.

The operation of the system when the image of the filter on the surfaceof the spatial light modulator is displaced will now be described.

For each color image to be generated, a starting position will bedefined for the displacement device from among all the possiblepositions (for example for an excursion of ±2 pixels in each direction25 positions are possible, 9 positions for ±1 pixel in each direction).This position will generate the first sub-frame by imaging red, greenand blue pixels through the patterns of the filter 3.

Let us assume that the filter 3 is embodied through the assemblage offour patterns such as that of FIG. 5 e. FIG. 5 f represents the image ofthe filter on the entrance face of the spatial light modulator.

Let us assume that we were to observe the position X of the image ofFIG. 5 f (column 7 and row 8 of the image).

During the first sub-frame, this position is illuminated in red.

The following two sub-frames will have to be illuminated afterdisplacements complying with the shape of FIG. 5 a so that the majorityof the positions of the spatial light modulator are illuminated by thethree colors. For example, for the second sub-frame, the pattern willhave to be translated by an elementary filter leftward so that a greenelementary filter (row 8, column 8) illuminates the position X of themodulator. Thereafter, for the third sub-frame, it is the blueelementary filter of row 7 and of column 8 which will illuminate theposition X, doing so through a translation of an elementary filterdownward. The shapes of FIG. 5 a being distributed regularly in thepattern of FIG. 5 b and subsequently, in the filter of FIG. 5 c, it istherefore seen that all the positions such as X of the spatial lightmodulator will have been illuminated by red, green and blue light aftertwo displacements of the image of the filter over the surface of thespatial light modulator. If all the pixels of the spatial lightmodulator are on during the whole of this sequence, the observer thenobserves a light transmitted by the modulator which is the combinationof red, of green and of blue and which is therefore white.

In certain cases, it is noted that it may happen that the fact ofhaving, in the filter 3, neighboring elementary filters of like colorleads to having, after three displacements during three sub-frames, animage which is not perfectly white. To remedy this, provision is made torebalance the colorimetry by three additional displacements during thethree subsequent sub-frames. FIGS. 6 a to 6 l illustrate this operation.

FIG. 6 a represents the image of the filter on the useful part of thespatial light modulator in the form of a matrix of numbers. Each numberrepresents a color:

-   -   a “1” represents blue;    -   a “10” represents green;    -   a “100” represents red.

In what follows, the units numeral will represent blue, the tens numeralgreen and the hundreds numeral red. This implies that a pointrepresented by a number 110, for example, will contain red color andgreen color but will not contain blue.

The image of the filter of FIG. 6 a is projected onto the entrance faceof the spatial light modulator at a determined instant while it is in adetermined position x=0 and y=0. It is assumed that all the pixels ofthe modulator are on. FIG. 6 b represents the image that ought to beperceived by an observer who observes the image displayed by the spatiallight modulator. This image is for the moment that of the image of thefilter. In particular the point of row 8 and of column 8 has the value10 (green).

The image of the filter will be displaced in such a way as to describe ablock shape such as represented in FIG. 5 a.

In FIG. 6 c, the image of the filter is displaced by one pitch leftward(x=1 and y=0). An observer ought to perceive, in FIG. 6 d, thesuperposition of the image of FIG. 6 b and the image of FIG. 6 c. Forexample, the point of row 8 and of column 8 has the value 110 and heought to perceive a superposition of red and green, i.e. yellow.

In FIG. 6 e, the image of the filter is displaced by one pitch downward(x=1 and y=1). An observer ought to perceive the superposition of theimage of FIG. 6 d and the image of FIG. 6 e. This is represented by FIG.6 f. For example, the point of row 8 and of column 8 has the value 210and he ought to perceive a superposition of red and of green, the redthen being twice as intense as the green, i.e. an orange color.

The integration of the various images viewed by the observer does notgive a white image. In particular; for example, it may be seen that thepoint of row 8, column 8 comprises no blue color and comprises red thatis twice as intense as the green.

The image of the filter will therefore be displaced again so as to makeit describe a shape such as that of FIG. 5 a.

In FIG. 6 g, the image of the filter is displaced, for example by threepitches leftward for example (x=4 and y=1). An observer ought toperceive, in FIG. 6 h, the superposition of the image of FIG. 6 f andthe image of FIG. 6 g. For example, the point of row 8 and of column 8has the value 220 and he ought to perceive a superposition of red and ofgreen (yellow color).

In FIG. 6 i, the image of the filter is displaced thereafter by onepitch downward (x=4 and y=0) An observer ought to perceive, in FIG. 6 j,the superposition of the image of FIG. 6 i and of the image of 6 h. Thepoint of row 8 and of column 8 has the value 221 and the observer oughtto perceive a superposition of red, of green and of blue, with a weakerintensity of blue.

In FIG. 6 k, the image of the filter is displaced finally by one pitchrightward (x=3 and y=0). An observer ought to perceive, in FIG. 6 l, thesuperposition of the image of FIG. 6 k and the image of FIG. 6 j. Thepoint of row 8 and of column 8 has the value 222. After integration ofthe various images produced in the course of the various precedingdisplacements, the observer therefore perceives a white light at thepoint of row 8, column 8. By analyzing the behavior of the variouspoints of the spatial light modulator it would be noted that the sameholds for all the points. The observer therefore perceives a spatiallight modulator which emits a light that is uniformly white on average(all the pixels of the modulator obviously being on as assumed earlier).

In the exemplary embodiment above, the following sequence ofdisplacements of the image of the filter has been effected:

dx dy 0 0 1 0 1 1 4 or −2 1 4 0 3 0

Other sequences of displacements may be selected so as to have a whitecolorimetry of the spatial light modulator when the pixels of the latterare on. The invention therefore makes provision to establish a selectionof these sequences of displacements and to give each of them theircharacteristics such as the position of the origin of displacement andthe types of displacements along two coordinates, X and Y. Thereafter,the invention makes provision to choose a sequence of displacements ateach frame. The sequence of displacements may be different from oneframe to the next, but this may well not be systematic and be decidedrandomly.

To establish this selection it is possible, for example, on the basis ofthe previous sequence of displacements, to deduce a following sequenceby translation of +1, +1. The sequence which follows is therefore avalid sequence:

dx dy 1 1 2 1 2 2 5 or −1 2 5 1 4 1

Another method for obtaining other valid sequences of displacements isto permute the orders of displacements within one and the same sequence.This amounts for example to permuting the first three points of theabove path among one another and to permuting the last three points ofthe above path among one another. The sequence deduced from the firstsequence described earlier is thus obtained:

dx dy 1 0 0 0 1 1 4 or −2 0 3 0 4 1

Other valid sequences may be found by other methods.

The control device 5 will control the changes of sequence ofdisplacements. These changes will preferably take place between twoimage frames.

Provision may be made to supply the control device 5 with a list ofsequences of displacements each independently making it possible toobtain a white colorimetry of the image of the filter. The device willchoose in a predetermined manner, or in a random manner, the sequencesof displacements to be used.

For the application of operation in which the white colorimetry isobtained after several sequences of displacements, the control device 5will be supplied with a list of combinations of sequences making itpossible to obtain a white colorimetry of the image of the filter. Inthis case of combinations of sequences, preferably, each sequence ofdisplacements taken on its own will not make it possible to obtain awhite colorimetry, and this will be useful in combating piracy, with theaid of a camcorder, of the images displayed by the spatial lightmodulator, as will be clarified hereinafter.

A random combination of all these valid paths will therefore allow a“coding” of the images according to colors, doing so according tononrepetitive sequences. This coding will not be easily decodable by apirate, all the more so since the camcorder will have carried out aresampling and a spatial and temporal averaging thereof.

The disturbance imparted to the video signal appears when there is nocorrespondence between the sampling time of a camcorder and the displaytime for sub-frames.

In the case identical to that described earlier where a sequence ofdisplacements of the filter is spread over two sequences so as to formsix consecutive sub-frames organized in such a way that the signalintegrated over these sub-frames is white (when the totality of thespatial light modulator is on), and in the case where the acquisition bya camcorder is done over three sub-frames only which straddle the sixsub-frames, the video recording of the camcorder will mix two colorcodings and will therefore create visible artifacts as explained in theexample hereinafter and illustrated by FIGS. 7 a to 7 c.

To simplify the example it is assumed that the acquisition frequency islocked to the display frequency and that the offset is constant, equalto a sub-frame. In the example of sequences presented in the tablebelow, the three sub-frames a, b and c acquired by a camcorder for theintegration 1 do not represent the output state of the camcorder but theprogress of the temporal integration of the light signal. the outputimage is the third sub-frame (sub-frame c for integration 1). In anexemplary operation that is clarified by FIGS. 7 a to 7 c, the displayhas been effected using the following sequences of displacements:

Sub-frame dx dy 0 0 A 1 0 B 1 1 {close oversize parenthesis} Integration1 C 4 1 4 0 {close oversize bracket} Integration 2 3 0

Then

dx dy 1 0 ] 0 0 1 1 {close oversize parenthesis} Integration 3 4 0 3 0{close oversize bracket} Integration 4 4 1

Then a sequence beginning with:

dx dy 1 0 ]

In the FIGS. 7 a to 7 c is represented the manner of operation relatingto a part of the filter of FIG. 5 f (the part situated at the top leftof FIG. 5 f). Just as for FIGS. 6 a to 6 l, the successive images of thefilter that are projected onto the spatial light modulator during eachdisplacement are represented in the left parts of these figures. In thecentral part of these figures is represented the integration of theimages on the spatial light modulator when these images correspond tothose of the right part of FIGS. 6 a to 6 l. In the right part arerepresented the integrations preformed by a camcorder filming theseimages.

As mentioned before, the integration 1 by the camcorder is out of phasewith respect to the projected images. It is noted, in these figures,that the results of the integration do not therefore correspond toexpectations. In FIGS. 7 b and 7 c, it may be seen in particular thatintegrations 2, 3 and 4 are far from giving a white field. Inintegrations 3 and 4 in particular (images 9 and 12), it is noted thatthe proportion of white pixels (level 111) is only 22%, the other pixelsbeing colored. This occurs for a uniform illumination and all the pixelsof the spatial light modulator being on. It is therefore necessary toadd to these defects those related to the changes of images which occur.Specifically, images 2, 5, 8 and 11 are each obtained after cycles ofthree sub-frames. Between images 2 and 3, 5 and 6, 8 and 9, 11 and 12,there are therefore changes of images and the integrations by thecamcorder will systematically integrate mutually different images, andthis will impair the quality of the image.

Referring to FIGS. 8 a to 8 c, a variant embodiment of the filter of theinvention will now be described. This variant relates to the embodyingof a filter with the aid of simpler blocks such as is represented inFIG. 8 a. This arrangement reduces in particular the proximity of blocksof like color. It is obtained by juxtaposing linear patches where thethree colors R, G and B are aligned. The displacement of the filter isadvantageously done in a single direction, along X or Y, and threesub-frames are sufficient here to attain the white state (see FIG. 8 b).

The diagonal alignment of the colors may turn out to be detrimental tothe viewing. This may be countered by advantageously interchanging pairsof rows or columns so as to scramble the pattern while avoidingjuxtaposing the same color twice.

Thus in FIG. 8 c, columns 4 and 5 and rows 4 and 5 have beeninterchanged.

It should be noted that the system of the invention is applicable tosystems providing an intermediate display between the source and thespatial light modulator 2 and making it possible to supply a relayimage. In this case the filter 3 may advantageously be associated withthis intermediate display.

1. A system for displaying images with the aid of a spatial lightmodulator wherein the system comprises: a light source emitting anillumination beam; the spatial light modulator comprising a matrix ofpixels controlled by video control signals corresponding to a successionof image frames to be displayed; a matrix filter formed of a mosaic ofadjacent elementary filters of various colors, illuminated by saidillumination beam and transmitting a spatially filtered color beam tothe spatial light modulator, means for producing an image of said filteron an entrance face of the spatial light modulator; means ofdisplacement for displacing said image of the filter on the entranceface of the spatial light modulator and a device for controlling thesemeans of displacement, making it possible to control at least onesequence of displacements of the image of the filter during each imageframe.
 2. The system for displaying images as claimed in claim 1,wherein the dimensions and the position of each elementary filter areadapted so that the image of each of them on the entrance face of thespatial modulator covers a plurality of pixels.
 3. The system fordisplaying images as claimed in claim 2, wherein each displacement of asequence corresponds to a multiple of the dimension of the image of anelementary filter on the entrance face of the spatial modulator.
 4. Thesystem for displaying images as claimed in claim 3, wherein said mosaicis monodimensional and includes only one column of elementary filters ofvarious colors.
 5. The system for displaying images as claimed in claim3, wherein said mosaic is bidimensional and in that said elementaryfilters are arranged in several rows and several columns.
 6. The systemfor displaying images as claimed in claim 5, wherein said mosaic isformed by the repetition of blocks of elementary filters, and in thatthese blocks exhibit identical contours and are each composed of atleast two elementary filters of different colors.
 7. The system fordisplaying images as claimed in claim 6, wherein said mosaic is anassemblage of identical patterns each comprising the same number ofblocks and the same number of elementary filters of each color in eachof the rows and in each of the columns of said pattern.
 8. The systemfor displaying images as claimed in claim 6, wherein each sequence ofdisplacements of the image of the filter on the entrance face of thespatial light modulator allows the successive illumination of each pixelof the spatial light modulator by all the elementary filters of one andthe same block.
 9. The system for displaying images as claimed in claim8, wherein, during each image frame, each pixel of the spatial lightmodulator is illuminated successively by all the elementary filters of afirst block under the effect of a first sequence of displacements, thenby all the elementary filters of at least one second block under theeffect of at least one second sequence of displacements.
 10. The systemfor displaying images as claimed in claim 5, wherein all the sequencesof displacements controlled by said control device are adapted so thatthe integration of the images of the filter that are obtained over theset of displacements of the sequence or sequences of each frame impartsa white colorimetry to the entrance face of the spatial light modulator.11. The system for displaying images as claimed in claim 10, whereinsaid first and at least second sequences of displacements are adapted sothat the integration of the images of the filter that are obtained overthe set of displacements of any one of these sequences imparts anonwhite colorimetry to the entrance face of the spatial lightmodulator.
 12. The system for displaying images as claimed in claim 10,wherein said control device possesses the characteristics of a pluralityof different sequences of displacements making it possible to impart awhite colorimetry to the entrance face of the spatial light modulatorand in that it selects, from among this plurality, different sequencesfor successive frames.